1. Field of the Invention
The present invention is directed to a gradient coil assembly for use in a magnetic resonance imaging apparatus for obtaining tomograms of an examination subject, such as a human body whose longitudinal body axis is oriented along the x-axis of a rectangular coordinate system, and whose body region to be examined is situated between the pole pieces of a fundamental field magnet having a fundamental magnetic field which extends in the direction of the z-axis, and in particular to such an assembly wherein gradient coils are in the form of flat coils arranged in respective planes parallel to the pole surfaces.
2. Description of the Prior Art
Magnetic resonance imaging (MRI, or nuclear magnetic resonance (NMR)) tomography devices construct an image from the spatial spin density, or from the distribution of relaxation times, of nuclei in a body under examination. This is accomplished by computational or mensurational analysis of integral proton resonance signals. Many types of tomograms can be produced, and such devices can also be used for joint diagnostics as well as for portraying blood vessel. The examination subject is introduced into a strong, uniform magnetic field, known as the fundamental field, which aligns nuclear spins in the subject. Pulsed gradient coils are provided which generate a number of spatially different gradient magnetic fields for topical resolution in the imaging volume. A radio-frequency antenna excites the nuclear spins, and also receives the resonance signals emitted by the excited nuclei.
It is known to employ superconducting magnets for generating the fundamental field. Such superconducting magnets are used to generate stronger magnetic fields, above approximately 0.5 T, and are in the form of solenoids which generate a static fundamental field proceeding along the longitudinal body axis of the patient. It is also possible to employ pole piece magnets which may be in the form of so-called C-magnets, H-magnets or window frame magnets. If such pole piece magnets are used in a magnetic resonance imaging apparatus, the longitudinal body axis of the patient is oriented in the direction of the x-axis of a rectangular coordinate system. The body region to be examined is situated between the ferromagnetic pole pieces of the magnet, whose magnetic axis extends in the direction z-axis. The pole piece magnet can be excited electromagnetically, or by a permanent magnet, as disclosed in German AS 3,737,133. For homogenizing the fundametal field by correcting for inhomogeneities occurring at the magnet and gradient coil edges, it is known to dispose correction coils in the air gap of an electromagnet, the magnetic field generated by the correction coils being superimposed on the field generated by the electromagnet. The correction coils are usually flat (pancake) coils. The flat coils are arranged immediately in front of the planar pole pieces of the magnet, as described in German AS 1,107,824.
Another embodiment of a magnetic resonance imaging apparatus is described in European application 0,178,216. In this apparatus the body axis of the patient extends in the x-direction and the fundamental magnetic field B.sub.0 extends in the z-direction. A special arrangement of pulse gradient coils is provided based on the object of generating an optimally constant gradient in each of the x-direction, the y-direction and the z-direction, with which a topical resolution is achieved for the purpose of imaging. The ideal, linear gradient fields for coils having an infinite expanse on the pole surfaces have the form ##EQU1##
The coils provided for the z-gradient are in the form of flat solenoids, and arranged on the pole surfaces of the fundamental field magnet. The turns of this coil are connected in series and form groups whose number of turns increases step-by-step with increasing distance from the center of the coil. The current proceeds in opposite directions in the two flat coils. The coils for the x-gradient and y-gradient are each composed of a series circuit of turns which form two conductive layers of conductors parallel to each other on the pole surfaces, and which are permeated by current in same direction. The coils for the y-gradient are disposed 90.degree. relative to the x-gradient coils, so that the x-gradient coils and the y-gradient coils from a system of orthogonal conductors.